This invention relates to a method and apparatus for processing digital signals and, more particularly, is directed to a method and apparatus for detecting and correcting errors in digital signals which are recorded and reproduced with a digital video tape recorder.
Recently, digital techniques have been used for the transmission and recording of video signals. In particular, a rotary head type video tape recorder (VTR) has been used to record pulse code modulated (PCM) video signals on a magnetic tape and, upon reproduction by the rotary head, the video signals are pulse code demodulated. In such case, the digital video signals are generally grouped into blocks with each block containing a predetermined number of bits. Upon reproduction, each block of the reproduced digital video signals is processed.
However, when a PCM-encoded video signal is recorded and subsequently produced, there is the possibility that the reproduced video signals may contain random errors caused by various types of noise, such as head noise, tape noise and amplifier noise, and burst errors (signal drop-out) resulting from dust or flaws on the tape surface. It should be appreciated, of course, that such errors may seriously deteriorate the quality of the video reproduction. In order to minimize this problem, error correction codes have been used in encoding the PCM signals prior to recording on the tape. For example, parity words may be added every predetermined number of blocks of video data and such parity words are then used during reproduction in an error detection operation. By using such error correction codes, erroneous PCM signals which are reproduced may be corrected or compensated so as to avoid the aforementioned deterioration in video reproduction. It should be appreciated that the more error correction code words that are used, the more accurate is the error detection/correction operation. However, it is also desirable, in achieving such error correction, to reduce the redundancy or number of error correction bits as much as possible so as to increase the area of tape that can be used for recording of data.
Further, when the number of errors becomes too high, so as to exceed the error correcting capability of the error correction code, an error concealment operation, rather than an error correction operation, is used. For example, the error concealment operation may be accomplished by replacing the erroneous video data with video data which is approximately equal thereto. In this regard, a field memory for storing successive fields of video data is provided and an address signal is added to each block of video data for addressing the blocks of video data into the field memory. When the speed of movement of the magnetic tape during reproduction is faster than that during recording, the rotary head is shifted so as to skip over a predetermined number of tracks to reproduce, for example, every other track. During reproduction at a speed slower than that of recording, the rotary head scans the same track more than once and then jumps over to the next adjacent track. As a result, the reproduced video data is not of a continuous nature. In this regard, the address signals of the reproduced video data are used to write the video information into the field memory at predetermined addresses so as to obtain a picture having continuity.
When the aforementioned error concealment operation is utilized with a digital color video signal, the phase of the color sub-carrier may be inverted at the connection point between the original erroneous video data and the substituted video data. More particularly, in the case of an NTSC system, the phase of the color sub-carrier between corresponding portions of successive frames differs by .pi./2. It should therefore be appreciated that when video data from one frame is substituted for corresponding video information of a successive frame, the phase of the color sub-carrier of the substituted video information must be inverted to provide a continuous phase relation of the sub-carrier. This is explained more fully in U.S. Patent Application Ser. No. 06/194,830, filed Oct. 7, 1980, having a common assignee herewith. In this regard, it has been proposed to add an identification signal to the video data for indicating the frame, field and line to which the video information belongs. However, if an error results in the identification signal, such phase inversion cannot readily be performed.
Further, to more accurately correct any error caused by drop-out, it has been proposed to add still another error correction code to the video data signal to more accurately detect and correct any error produced in each block of video data. In such case, it is desirable to use a code having a high capability of error detection and correction while, at the same time, having a low redundancy.